KatieeeBethhh16: "13+20+27+ ... +272"
[input]sum ( 13+x*7, x=0..((272-13)/7) )[/input]
this is your series:
13+20+27+ ... +272
this is the series in reversed order:
272+265+258...+13
the sum of one element of the series and one element of the reversed series (at the same index) is always the same:
13+272=285
20+265=285
27+258=285
... = 285
you have (272-13)/7 +1 =
38 such terms.
then the sum of 2 times the arithmetic series is: 38 * 285 =
10830 the sum of the arithmetic series "13+20+27+ ... +272" is then: 10830/ 2 =
5415
+3146 
